1. Field of the Invention
The present invention relates to a method and an apparatus for cooling a metal strip in a continuous annealing furnace. More particularly, the present invention relates to a cooling method and apparatus in which a metal strip which is heated in a continuous annealing furnace is cooled by bringing it into contact with a cooled roll, the metal strip being uniformly cooled by this method and apparatus at a predetermined cooling rate as seen in the short width direction thereof.
2. Description of the Prior Art
Due to recently developed techniques for producing cold-rolled steel strips, strips exhibiting good drawability and formability can be produced by continuous annealing rather than batch annealing. Since the continuous annealing techniques enable a higher productivity and a greater cost reduction than do the batch annealing techniques, the continuous annealing techniques have been more broadly used heretofore.
Representative heat cycles, to which a steel strip is subjected in a continuous annealing furnace, are explained with reference to FIGS. 1 and 2.
In FIG. 1, the so-called stop quenching heat cycle is illustrated. In the primary cooling step, gas-jet cooling, in which a cooling gas is directly blown onto the heated steel strip, is carried out. In FIG. 2, the so-called full quenching heat cycle is illustrated. In the primary cooling step, the heated steel strip is cooled by spraying it with water and then immersing it in water.
The above-described cooling methods are disadvantageous in the following respects:
(1) By gas-jet cooling, it is difficult to attain a high cooling rate of approximately 100.degree. C./second.
(2) Water-cooling can provide a high cooling rate but cannot realize an end-point control; thus, the steel strip is inevitably cooled to normal temperature. The steel strip must, therefore, be reheated up to a temperature at which overaging is carried out. In addition, since the surface of the steel strip is oxidized, a treatment such as pickling becomes necessary and, thus, the installation and operation costs become high.
As one method by which the above-described disadvantages can be eliminated, a roll-cooling method is proposed. In the roll-cooling method, a heated steel strip to which a predetermined tension is applied is engaged with and is turned around a rotatable roll which is continuously cooled by a cooling medium. The known roll-cooling methods cannot attain uniform cooling of a steel strip as seen in the short width direction for the following reasons.
The roll body of a cooling roll is monolithic and has such a length that a steel strip is brought into contact with a central portion of the roll body as seen in its axial direction. The temperature of this central portion is higher than the non-contact portion, with the result that a heat crown is formed on the roll body and, thus, contact between the steel strip and the roll body is impeded at both edges of the steel strip. Both edges of the steel strip are, therefore, not cooled, and a nonuniform temperature distribution is generated along the short width direction of the steel strip. The cooled central portion of the steel strip shrinks thermally, with the result that a nonuniform tension, i.e., a high tension at the cooled central portion and a low tension at the non-cooled edges, is generated in the steel strip. Due to this nonuniform tension distribution, the cooled portion of the steel strip having a high tension is further brought into close contact with the cooling roll, and contact between the non-cooled portions (both edges) having a low tension with respect to the cooling roll is further impeded. As a result, the nonuniform temperature distribution along the short width direction of the steel strip, i.e. a direction traversal to the strip conveying direction, is intensified.
Japanese Unexamined Patent Publication No. 57-23036 proposes the use of a cooling medium capable of being used at a higher temperature than the water and thus decreasing the temperature difference between the cooling roll and the steel strip, thereby attempting to lessen the nonuniform temperature distribution as seen in the axial direction of the roll body. However, according to this proposal, the cooling efficiency in terms of the cooling rate of a steel strip becomes considerably less than that attained by the roll-cooling method in which water is used. In addition, according to this proposal, effective reduction of the heat crown is not provided.
Japanese Unexamined Patent Publication No. 54-118315 proposes the formation of, in the roll body of a cooling roll, cooling medium passages which are separated from each other as seen in the axial direction of the roll body. It also proposes separate control of the cooling medium temperature and the flow rate in each cooling medium passage. In the cooling method proposed in this publication, it is difficult to optionally reduce the cooling rate in a selected cooling medium passage(s) since bumping must be prevented. Therefore, the heat crown cannot be reduced to a satisfactorily low level.
According to the proposals of the above two publications, the structure of the cooling roll becomes complicated, and the controlling device becomes largesized and uneconomical.